A typical structure of a fuel assembly for a pressurized water reactor is shown in FIG. 5. Describing it briefly, an upper and lower nozzle 1, 3 having a plurality of coolant flow holes machined therein are connected with a plurality of hollow guide tubes 5 which are parallel to one another. Support grids 7 having grid cells positioned in a rectangular arrangement which individually receive the hollow guide tubes 5 therethrough and which are fixed to the guide tubes 5 support the fuel rods 9 by placing them individually through the remainder of the grid cells. A fuel assembly 10 is thus constructed and in order to more clearly show the structure, a portion of the fuel assembly 10 from which the surroundings about one support grid 7 are removed is shown in a partial perspective view in FIG. 6. As readily understandable from FIG. 6, the disposition of the fuel rods 9 is in a square arrangement with equal numbers of columns and rows and it is constructed showing some of the fuel rods 9 at specified locations being replaced with the hollow guide tubes 5.
The support grid 7 defining grid cells which individually receive, as described before, the fuel rods 9 and the hollow guide tubes 5 disposed in a square arrangement, is essentially constructed by combining two kinds of thin metal straps as shown in FIGS. 7a and 7b, namely, straps 20, 30, with each other in a perpendicular relationship. The support grid 7 defines grid cells located in a 14.times.14 arrangement, and the straps 20, 30 each essentially have an identical configuration and differ from each other in the positions at which slits 21, 31 for receiving another strap corresponding thereto are formed. In other words, slits 21 in the strap 20 are positioned on an upper side (downstream in the coolant flow), while slits 31 in the strap 30 are positioned on a lower side (upstream in the coolant flow). Mixing vanes 23, 25, 33, 35 are integrally formed at a downstream edge of the straps 20, 30 in alignment with the slits 21, 31. Furthermore, tabs 27, 29, 37, 39 for welding the straps 20, 30 which are assembled by using slits 21, 31 are provided. The positional relationship among the slits 21, 31, the mixing vanes 23, 25, 33, 35 and the tabs 27, 29, 37, 39 that are described above is schematically shown in an enlarged manner in FIGS. 5a and 8b.
The above described mixing vanes 23, 25 and 33, 35 are bent and slanted in opposite directions, respectively, as shown in FIG. 9, when the straps 20, 30 are assembled. A partial top view of the support grid 7 corresponding to FIG. 9 is shown in FIG. 10. As seen from FIG. 10, the outer extremities of the mixing vanes 23, 25, 33, 35 are close to the fuel rods 9 depicted by dash-and-two-dot lines, but not close enough to come into contact with the fuel rods 9. In these structures, since a coolant stream which flows through the support grid 7 impinges on the mixing vanes 23, 25, 33, 55, the coolant stream is agitated to be stirred and make the temperature distribution therein uniform.